Neuropathic pain is believed to result from sensitization reactions in the peripheral and central nervous system. Such pain can occur as a result of peripheral injuries, or as a result of systemic diseases such as HIV, herpes zoster, syphilis, diabetes and autoimmune diseases. Neuropathic pain can be severe and is often debilitating, and effective methods for reducing neuropathic pain would ameliorate significant suffering.
In U.S. Pat. No. 6,248,788 (Robbins et al.), a topical method of treatment of neuropathic pain with capsaicin or substances analogous to capsaicin is described. The Robbins et al. patent disclosed that treatment of the affected body areas once or at most twice with a highly concentrated capsaicin preparation for a few hours eliminates or significantly alleviates the pain for a number of weeks. It is believed the basis for this treatment is that the nerve fibers necessary or responsible for the pain sensation (C fibers) are desensitized by the capsaicin (or capsaicin analog) and degenerate. However, this effect only occurs when the active compound concentration in the C fibers is high enough. Conventional topical preparations containing capsaicin do not optimally fulfill these requirements, as they release too little capsaicin on the skin and the active compound concentration in the C fibers remains below the effective concentration.
U.S. Pat. No. 6,239,180 (Robbins) describes the use of therapeutic patches comprising capsaicin and/or a capsaicin analog at a concentration of greater than 5% to 10% by weight for treatment of neuropathic pain. The object was thus to develop a patch which is suitable and optimized for the topical therapy of neuropathic pain and other conditions.